The present invention relates generally to the removal of pollutants from flue gases and more particularly to a method and apparatus for removing sulfur dioxide from flue gases exhausted from boilers fired with sulfur-containing fuel.
Sulfur-containing fuels, such as coal, coke oven gas or fuel oil, are typically used to fire boilers for producing steam to generate electricity and/or for heating o processing purposes. Typically, the fuel is combusted with air, in excess of the stoichiometric amount required for combustion, at a series of burners in an enclosed combustion chamber to produce combustion reaction products consisting primarily of hot gases but also containing some particulates, such as fly ash. Heat is extracted from the hot gases, in a conventional manner, to heat water and produce steam. The hot gases are flowed in a downstream direction and eventually are exhausted through a stack. Residual heat remaining in the hot gases, after completion of the steam producing function, may be used to preheat combustion air.
In the combustion chamber, the temperature decreases in a downstream direction after the last burner. Moreover, at any location along the downstream path there can be a spread of different processing temperatures across the lateral dimensions of the combustion zone. However, at any such location. There is also an average temperature, and the average temperature is the temperature reference used herein, unless otherwise indicated.
The hot gases from the combustion reaction include undesirable pollutants, both solid and gaseous. Solid particulate pollutants are usually removed in an electrostatic precipitator or a bag house or both. Gaseous pollutants have included oxides of nitrogen (NO.sub.x) and sulfur dioxide (SO.sub.2). Within the last few years, the NO.sub.x content of the gases has been reduced by changes in combustion techniques for oil and gas-fired boilers and by changes in burner design for coal-fired boilers.
A high sulfur dioxide content in the gases is especially undesirable because, if allowed to escape into the atmosphere, it can be a source of acid rain as well as other undesirable effects.
Attempts have been made to reduce the sulfur dioxide content of the combustion reaction gases (flue gases) by an expedient known as dry sorbent injection. A sorbent is a compound which reacts with the sulfur dioxide to produce a relatively innocuous, solid compound which can be removed from the flue gases with conventional particulate removal apparatus. Examples of dry injection sorbent materials previously employed to remove sulfur dioxide from flue gases resulting from the combustion of coal include the carbonates or hydroxides of magnesium and calcium. Limestone (calcium carbonate) particles have been employed as a dry sorbent injection material in coal fired boilers. In such a system, the sulfur dioxide in the flue gases is converted to calcium sulfate, an innocuous solid compound which can be employed as a construction material or which may be buried in a land fill without concern for adverse effects on the environment. Initially, the particles of limestone or calcium carbonate (CaCO.sub.3) are calcined into lime (CaO) by the heat from the combustion reaction, and the lime reacts with the sulfur dioxide, in the presence of oxygen (from the excess air in the combustion chamber) to produce calcium sulfate (CaSO.sub.4).
As noted above, oxides of nitrogen in the flue gases have been reduced by employing an improved burner design. Such a burner design generally includes a nozzle through which the fuel is injected into the combustion chamber together with so-called primary air. Also injected into the combustion chamber, at locations closely adjacent the fuel nozzle, is secondary air which, together with the primary air, accounts for about 0.7-1.0 times the stoichiometric amount of oxygen required for complete combustion. In addition to the primary and secondary air, tertiary air is also injected into the combustion chamber from locations either closely surrounding, or remotely spaced in a downstream direction from, the inlets for the secondary air.
Employing a burner arrangement of the type described in the preceding paragraph reduces or eliminates peak flame temperatures, the presence of which accounts for oxides of nitrogen produced from nitrogen in the combustion air. The aforementioned burner arrangement also reduces the oxygen concentration in the pyrolysis or chemical reaction zone of the flame, which controls the formation of oxides of nitrogen from the nitrogen contained in the fuel.
Attempts have been made by others, at least on a test basis, to inject limestone particles in a system employing burners of the type producing a low percentage of oxides of nitrogen, hereinafter referred to as low NO.sub.x burners. In these attempts, limestone has been injected into the combustion chamber through the fuel nozzles, through the inlets for introducing the secondary air (located closely adjacent the fuel nozzle), through tertiary air inlets closely surrounding the inlets for the secondary air, and through separate limestone-injecting inlets spaced relatively far downstream from the inlets for the fuel and the combustion air. In the first three instances, the limestone was premixed with the fuel and/or the combustion air entering the combustion chamber at the secondary and tertiary air inlets.
There are drawbacks to all of the limestone injection techniques described in the preceding paragraph. Injection through the fuel nozzle or through secondary air inlets immediately adjacent the fuel nozzle or through tertiary air inlets closely surrounding the secondary air inlets subjects the limestone particles to relatively high temperatures for relatively long periods of time, and this can cause sintering of the resulting lime particles which reduces their surface area and therefore their ability to react with SO.sub.2, thereby decreasing SO.sub.2 removal. Introducing the limestone particles relatively far downstream from the fuel nozzles and combustion air inlets decreases SO.sub.2 removal because the temperature conditions are too low and/or decrease too rapidly.
Premixing the limestone particles with the combustion air causes erosion and pluggage problems in the transporting conduits for the combustion air and reduces substantially the accuracy with which the particles can be divided among the substreams to the individual air outlets, a multiplicity of which are usually employed. These problems arise from the high velocity at which the combustion air flows through the transporting conduits, e.g. 2,500-5,000 ft/min. (762-1524 m/min.) and the fact that the limestone particles are carried in dilute phase transport.
If the velocity of the transporting combustion air is reduced to decrease the erosion and pluggage problems, the volume of the transporting air has to be increased in order to carry the limestone in dilute phase transport at the slower speeds; and this could result in more combustion air at a given outlet, or series of outlets, than is desired from the standpoint of combustion or other considerations. Moreover, lowering the velocity at which the combustion air is introduced into the combustion chamber reduces the turbulence and mixing action due to the combustion air, and such a reduction is undesirable. Furthermore, a minimum velocity is necessary in order for the combustion air to properly distribute within the combustion chamber the limestone particles carried by the combustion air.
The use of a large volume of air to transport and/or introduce the limestone particles into the combustion chamber has a substantial adverse effect on the ability of an electrostatic precipitator to remove particulates from the combustion reaction gases. This is because the electrostatic precipitator is usually sized for a given volume of air, i.e. only the air volume normally employed for combustion purposes (e.g. primary, secondary and tertiary air in a low NO.sub.x burner system). When a substantial additional volume of air is present as a result of limestone particle transport and introduction, an electrostatic precipitator, sized to handle a smaller volume, will not function properly.